The Great Bras d'Or (Seal Island) Bridge was constructed in 1960-61 and opened to traffic in 1962. The bridge is located on Highway 105 in Cape Breton, N.S., a vital stretch of the Trans-Canada Highway leading to the Newfoundland Ferry Terminal at North Sydney and is the only crossing of Great Bras d'Or. The structure consists of eight steel box truss spans; three simply supported 250' approach spans, two simply-supported 250' splay spans, and a three-span continuous main span that consists of two 350' side spans and a 500' centre arch span.

The existing cast-in-place bridge deck is approximately 75% delaminated and a number of significant through-slab holes have recently developed in all spans, most notably in the west approach span. It was therefore decided that a major deck replacement strategy was necessary.

In August 2001, the design of the deck replacement and collateral truss reinforcing of the west approach span (demonstration span) was completed and a call for tenders was made by the NSDOT for construction of the first span. As of August 2002, construction of the remaining four approach and splay spans is well underway; the design of the side spans and main arch span is almost complete. Tenders for construction of the main and side spans are scheduled for early 2003 with construction anticipated in the 2003-construction season.

Deck Replacement Concept

The concept involved the replacement of the existing cast-in-place concrete deck with full-depth, precast, prestressed, half-deck width concrete panels post-tensioned longitudinally to close the transverse joints and finally transversely once the centre closure strip had been cast. Following are the major features of the deck replacement concept:

•Factory produced high performance, precast concrete deck panels, prestressed in the transverse (main spanning) direction with distribution reinforcement provided in the longitudinal direction. The panels are subsequently post-tensioned in both directions. Composite action between the precast deck and longitudinal stringers is achieved by welding Nelson studs to the stringers through ports provided in the precast panels; the ports were grouted after the studs were in place and the post-tensioning completed.

•Precast cap, curb and barrier support units, anchored to the exterior edges of the deck panels; these precast units provide protection to the transverse post-tensioning anchorage zones, by encapsulating the edge of the precast panels to create a uniformly finished edge along each side. The units also stiffen the edge of the deck panels to resist traffic barrier impact loads.

The deck design was facilitated by a grillage model that incorporated all deck components and accounted for the stiffness in both the transverse and longitudinal directions, ensuring that the deck was fully prestressed in the transverse direction and that sufficient distribution reinforcing was present to satisfy the load requirements at ultimate limit states. The concrete mix selected is a 45 MPa HPC mix, with air entrainment, and strict permeability limits (maximum 1,000 coulombs at 90 days). The resulting deck is a crack-free design under service conditions.

Construction Methodology

The proposed construction sequence facilitated the retention of a driving lane while permitting the replacement of the other half of the deck. Removing the existing sidewalk enabled a sufficient width of the existing deck to be removed to provide space for the new deck components to be placed while providing a 10'-6" wide temporary driving lane.

Phase 1: Deck Replacement on the South Half of the Bridge

•A temporary steel barrier was installed on the north side of the deck, near the centreline of the new roadway, leaving a 10'-6" travel lane on the north side of the bridge.

•The existing deck was saw-cut and the south half of the deck was removed.

•The south panels were then installed.

•The transverse joints between the panels were grouted and the longitudinal tendons were stressed in a balanced fashion.

•The shear studs were then welded to the top flange of the stringers through the access ports provided in the deck panels and concrete was placed in the haunches and stud pockets.

•The south half of the new deck was completed by pouring high-performance cast-in-place concrete closure strips at the expansion joint. Finally, a silane sealer was applied to the top surface of the new deck to protect the concrete until the final wearing surface was applied.

Phase 2: Deck Replacement on the North Half of the Bridge

•The temporary barrier was repositioned to the south side of the bridge and the new traffic barriers were temporarily installed along the south edge of the deck using the permanent anchor bolts cast in the precast deck. The north half of the existing deck was then removed.

•The precast panels on the north half of the bridge were installed and longitudinally post-tensioned. The centre longitudinal joint between the north and south panels was then cast and the panels were transversely post-tensioned, prior to casting the concrete in the haunches and stud ports. The silane sealer was applied over the remainder of the deck top surface and the precast curbs and traffic barrier system were installed along each side of the new deck.

•A 1-1/2" thick temporary asphalt-wearing surface was placed to protect the deck on the west approach span through the winter season until the permanent Rosphalt 50 wearing surface could be applied and the plug joints installed during favourable weather conditions.

Discussion

The re-decking and strengthening of the West approach span of the Seal Island Bridge was completed within the anticipated construction schedule and within the estimated budget of $1.6 million.

In addition to the success of the project from a schedule and budget point of view, the demonstration span also verified the viability of the design concept and provided valuable construction insight that has since been utilized in the design of the remaining spans. Significant time and cost savings have been witnessed during the current contract.

Some other highlights of the project are:
•Construction was completed with minimal traffic disruptions and the bridge remained open to single lane traffic throughout construction, with the exception of three, six-hour overnight closures planned per week.

•During full-bridge closures, measures were put in place to allow for the uninterrupted passage of emergency vehicles.

•The new high performance precast concrete deck system is much more durable than conventional systems since it is less permeable and crack-free under service load conditions.

•The deck system adopted is significantly lighter than a conventional cast-in-place concrete deck system. This has resulted in considerable savings in the amount of truss reinforcement required, while providing sufficient mass and stiffness for damping purposes.